Discharge lamps produce light by ionizing a fill material, such as a mixture of metal halide and mercury in an inert gas, such as argon, with an arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel or discharge tube, which maintains the pressure of the energized fill material and allows the emitted light to pass through. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, including color temperatures, color rendering, and luminous efficiency.
Discharge tube chambers composed of fused silica “quartz” are readily formed. However, the lifetime of such lamps is often limited by the loss of the metal portion of the metal halide fill (typically sodium) during lamp operation. Sodium ions diffuse through, or react with, the fused silica discharge tube, resulting in a corresponding build-up of free halogen in the discharge tube. Quartz discharge tubes are relatively porous to sodium ions. During lamp operation, sodium passes from the hot plasma and through the discharge tube wall to the cooler region between the discharge tube and the outer jacket or envelope. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission. The light output consequently diminishes and the color shifts from white toward blue. The arc becomes constricted and, particularly in a horizontally operated lamp, may bow against the discharge tube wall and soften it. Also, loss of sodium causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained, ending the life of the lamp.
Ceramic discharge lamp chambers were developed to operate at higher temperatures than quartz, i.e., above 950° C., for improved color temperature, color rendering, and luminous efficacies, while significantly reducing reaction with the fill material. U.S. Pat. Nos. 5,424,609; 5,698,984; and 5,751,111 provide examples of such discharge tubes. While quartz discharge tubes are limited to operating temperatures of around 950° C. to 1000° C., due to reaction of the halide fill with the quartz, ceramic alumina discharge tubes are able capable of withstanding operating temperatures of 1000° C. to 1250° C. or higher. The higher operating temperatures provide better color rendering and high lamp efficiencies. Ceramic discharge tubes are less porous to sodium ions than quartz tubes and thus retain the metal within the lamp. Various techniques are available for fabricating the discharge tubes, including casting, forging, machining, and various powder processing methods, such as powder injection molding (PIM). In powder processing, a ceramic powder, such as alumina, is supported by a carrier fluid, such as a water-based solution, mixture of organic liquids, or molten polymers. The mixture can be made to emulate a liquid, a plastic, or a rigid solid, by controlling the type and amount of carrier and the ambient conditions (e.g., temperature).
The use of ceramic in high wattage metal halide lamps has improved the useful life and performance of such lamps. Nevertheless, ceramic metal halide lamps still suffer from progressively poorer light output (lumen maintenance) and color shift as the lamp ages and wattage is decreased. This makes it very difficult to manufacture a practical low wattage metal halide lamp having suitable performance.
In addition, typical low wattage ceramic metal halide lamps offer only marginal performance. For example, most 20 watt lamps suffer from such poor light output that their use in most commercial and personal applications are severely limited.
Thus, a need exists for a low wattage ceramic metal halide lamp that provides acceptable performance and lumen maintenance and exhibits minimal through-life color shift.